Milliamperage stabilizer



June 20, 1950 c, ZAVALES I 2,512,193

MILLIAMPERAGE STABILIZER Filed Sept. 20-, 1945 ATTORN EY Patented June20, 1950 UNITED STATES IHILLIAMPERAGE 'STABIIJIZER Charles T. Zavales,Baltimore, Add ,agssignorgto WestinghouseElectricrCorporation,East'Pittse burgh, Pa., a corporation ofPennsylvania Application September 20, 19,45,- Serial Nip--61 16,25

20 Claims. (Cl. 31 5-10Z) The present invention relates to X-ray systemsand more particularly to a milliamperage stabilizer system :formaintaining the current through the X-ray tube constant under allconditions.

It has long been recognized in the art that an X-ray tube is generallysubject to variations of several kinds during its operation which affectthe quality of the resulting radiographic exposure. In order toeliminatethe effect of some of these variables, numerous circuit arrangementshave been long utilized :in the art in an effort to stabilize theoperation of the X-ray tube. Practically all of such systems aredirected to merely maintaining the temperature of the thermionic cathodeof the X-ray tube constant by causing an inverse variation in thecathode heating current in response to line voltage fluctuations orvariations in the high voltage supplied to the X-ray tube.

Thereiare, however, other factors which cause a variation in the voltagesupplied to the cathode of the X-ray tube, or filament voltage as itwill be hereinafter referred to. Such variation in filament voltagewhich necessarily controls the temperature of the filament are heatingof the entirefilament circuit, changes in the characteristics of theX-ray tube and the change in kilovoltage across the X-ray tube. In allpresent type X-ray tubes employing filamentary. cathodes, the emissioncurrent is dependent on a definite.relationship which shows the emissioncurrent is dependent upon thetemperature of the filamentary cathode, buta rise in emission current is not proportionate to the actual filamentvoltage. Accordingly, as the voltage of the filament is raised, thequantity of emitted electrons rises greatly, thus necessitating somemeans of voltage stabilization ior the filament of the X-ray tube inorder to insure consistent tube milliamperages. 1 Such voltagestabilization of the filament does not compensate, however, for thecharacteristic of filamentary cathode X-ray tubes requiring that .foraparticular milliamperage setting, a decrease in actual filament heat isrequired in order to maintain the emission current constant as thekilovoltage across the X-ray tube is increased. Therefore, to meet thisrequirement it is essential toinsure proper decrements in X-ray filament,heat as the kilovoltage across the X-ray tube is increased.

As above mentioned, a further variable affect- .ingstabilization arisesfrom the fact that components-of theX-ray-filament circuit are inhera 2ently heated to some extent. Consequently, as the temperature of theX-ray filament circuit increases, due to :heating of its components, thefilament heat of the X-ray tube decreases .and continuedv operation ofthe X-ray tube withtconstant filament voltage across the primary of theX-ray filament transformer will result in continued diminution ofthe-tube milliamperage. It is accordingly the object of the present in.-vention to provide a milliamperage stabilizer for an X-ray tube whereincompensation is made for the rapid change ofemission characteristics ofthefilament as the kilovoltageacross the tube is increased.

Another Object of the presentiinvention is the provision of .amilliamperage stabilizer for an X-ray tube wherein compensation is madefor the .decreasecharacteristic in tube filament heat, required to,maintain constant tube current, as the,kilovoltage.across the tube. israised.

Anotherobjectofthe present invention is the provision of a,milliamperage stabilizer foran X-ray tube whereinthe effects of heatingin. the

, X-rayfilamentcircuit are'overcome so that the filamentarycathodeismaintained at a truevconstanttemperature.

vA further object .of the present invention is the provision ,of aQmilliamperage stabilizer for an X-ray tube wherein compensationis madefor substantially all variables which would otherwise tend to alter thecurrent flowing through the X-ray tube duringits operation.

Still further objects of the ,present invention .will ,become obviousto, those skilled in the art by reference to the accompanying drawingwherein the single figure is a diagrammatic illustration of .amilliamperage stabilizer constructed in accordance withthe presentinvention.

Referring .n,ow .,to the drawing in detail an X-l y tube 5 is shownhaving .its anode (i connected through a rectifying valve 1 to one endof thesplit secondary winding 8 ofahigh tension transformerfl, with theremaining end of this secondary winding ,Bgrounded at Hi. In a similarmanner the filamentarycathode 12.0: the X-ray tube 5 is connectedthrough a rectifying valve l3 to one end of the ,remaining portion of asec,- ondary winding I14 of ,the high tension transformer ,9.Theotheriendofthis secondary winding connects l-through variousinstrumentalities constituting thestabilizeras will be hereinafterexplained. Thefilamentary cathode .12 of the X-ray tube 5 receivesheating current ,from the secondary winding [5 Ma low tensiontransformer 16. a

auto-transformer H is provided with its primary being adjustablyconnected to a source of supply of the customary commercial potential of200 to 250 volts so that once adjusted for the available voltage, uponclosure of a magnetically operated switch l8, this voltage will besupplied to the auto-transformer II. It will be noted that theauto-transformer is provided with large steps throughout most of itslength comprising several turns of its winding between taps, for thepurpose of giving a voltage differential therebetween of ten volts,while the remainder of its winding is tapped at substantially eachindividual coil, for giving a voltage variation between taps in units.An additional winding 19 is carried by the core of the auto-transformerI1 which is also tapped to give a voltage variation in unitstherebetween.

An adjustable contact 2!] is operable to engage the various taps of theunit sectionofthe autotransforiner for selecting the desired units ofkilovoltage for the X-ray tube 5 whicharm is connected to one end of theadditional winding l9. One'en'd of the primary winding 22 of the,hig htension transformer 9 is adjustably connected to theadditional windingi9, so that the adjustment maybe altered to compensate for voltage dropsin the interconnecting cable, drops due to leakage reactance, andauto-transformer drop at each value of milliamperage andchange inlinevoltage conditions. The other end of this primary winding 22 iconnected, upon closure of a magnetically operated switch 23, to anadjustable arm.2 4 engageablewith the large ten .volt taps of theauto-transformer l! with the arms 20 and 2 1, thuspreselecting thedesiredkilovoltage forthe X-ray tube accurately in units and tens or 20kv. and 2 kv. steps. The magnetic winding of switch 23 receiveselectrical energy from taps of the auto transformer l1 upon closure ofapair of contacts 25 with the circuit to winding of switch zfiremainingclosed for a period of time determined by a timing device 26. Contacts25 are closed by a relay 21, as hereinafter described.

The primary winding 28 of low, tension heatingtransformer H3 receivesenergy ofthe proper voltage from auto-transformer I! by having one endof its winding connected directly to a. tap

of the auto-transformer'winding, while the remaining end of primarwinding. 28 is adjustably connected by an arm .29 to a resistance 30bridged across several windings of the auto-transformer 111, with aresistance 3! and the A. C. windings 33 and 3 of a saturable reactorbeing in series with the primary winding 2B..of filament heatingtransformer l6. e j H The saturable reactor 35 is provided in order tovary thefilament heat ofthe cathode ii to compensate for any inversevariations thereof due to decrease in line voltage, change inkilovoltage jor changeof resistance in the filament circuit by heatingof its components. In addition to its A. C. windings 33 and 34 disposedabout the outermost legsof the core, such reactor is provided with a, D.C. winding 35 wound about its centerleg. As is well known in the art, achange in magnitude of the current flowing in thecentral D. C. winding35 will vary the impedance 'of the two A. C. windings 33 and 34, i. e.,an increase in cure rent through the D. C. winding will decrease theeffective impedance of the two A. C. windings 33 and 34 allowingmorecurrent to flowto the pri mary of transformer it with an attendantincrease in filament heat and a decreasein current through the D. C.winding 35 will increase the impedance of the two A. C. windings with adecrease in filament heat of cathode [6.

If, therefore, the magnitude of the current flowing through the D. C.winding 36 is suitably controlled, the filament heat of the filamentarycathode i2 is conveniently varied to maintain substantially uniformheat. Accordingly, an electronic control tube 31 is provided which has arelatively high value of transconductance rather than a high voltagegain. By high value of transconductance is meant a tube, such asidentified in the trade as SAGl, which will vary the effective platecurrent in the greatest possible manner with the least possible changein grid voltage, since this tube type has the characteristics desired ina control circuit which is dependent on current rather than voltage as ameans of operation. Accordingly, the plate of tube 31 is connected toone end of D. C. winding 36 while its indirectlyheated cathodeisconnected to one side MAI of the input circuit of the stabilizer and tothe ne'gative side of a D. C. source of supply of about 350 volts, ashereinafter described, and the opposite end of the D. C. winding 36 isconnected to the positive side of this D. C. source of supply. Thesupply of a D. C. current at a voltage of about 350 volts to theD. C.winding 36 of the saturable reactor 35 is thus under the control oftube'3'l. A capacitorv M is shunted across the D. C. winding 36 toreduce the effects of self-induction which occurs due to rapid changesin plate cur rent through the tube 31. v

Referring now to high tension secondary winding M of transformer 9, itwill be noted that an end thereof is connected, through, a milliamperemeter 38 and shunted by a capacitor 39, to a milliamperage preselectorshown generally within the dotted frame 46. The latter constitutes aresistance type of control which increases or decreases the X-ray tubefilament current, thus raising or lowering the milliamperage flowingthrough the high voltage circuit of the X-ray tube. At the same timethis milliamperage preselector adjusts the current at which thestabilizer operates.

This milliamperage preselector comprises resistors R1, R2, R3, R4,together with previously mentioned resistance 30 connected across thetaps of auto-transformer i1. Resistors R1 to R4 select the magnitude ofthe voltage which is ap-, plied to the milliamperage stabilizer acrossthe terminals G1 and MAi. Resistance 39 controls the filament heat ofthe X-ray tube in such mannor as to increase the available filamentvoltage as the milliamperage is raised, with its adjustable arm 29 beingoperable in tandem (as shown by the legend) with the adjustable arm 42of resistor R1.

These two adjustable arms 29 and 42 are operable from the front of thecontrol panel and are employed in conjunction with a precalibratedmilliamperage scale. For the sake of accuracy, the high and low ends ofthe milliamperage selec tor scale is made to track with the selectorpointer, which is assured by the resistors R2, R3 and R4. Adjustableresistor R2 is provided to set the upper or high milliamperage to whichthe selector is adjusted while adjustable resistor R3 and fixed resistorR4 adjust the lower milliampere value on the precalibrated scale. Oncethese are set for a given installation, they need rarely be furtheradjusted with the simultaneously operable resistor R1 and the variableresistance 30 preselecting the milliamperage desired. I

This milliamperage preselector 40 operates by being supplied withcurrent from the milliampere amazes master the high tension itransformerwinding -I-'4- through-milliammeterMa and resistorsEz-arid R1 '--toground through ad-justable' arm-l2, and since the apper en'd oradjustable resistor' Rr is connected to stabilizer terminal- Ge andthe"lower end of resistor Rz is conneeted to stabilizer terminal 'Mnn adefinite controkvoltage is impressed across the mill-iampere stabilizer.A capacitor 01 is shunted across *the resistors Br and i to insure'proper tracking of the*calibrated milliam penage scale use to=waveform'=changes which occur in the milliampere-=circu-it athigher-values of *X raytzibe-current. *Aspreviously-mentioned,

resistors R3 and liware =prima1' ily employed to trim the lowmilliampere scale of the *precalimated-selector, =hutthey also play asecond role. Should *resistor ER-z become open-circuit'ec l',- due toany particular surge,=resistors *R3 and R4 insure the application er thecontrol voltage to the G1 andMAr terminals or the stabilizer.

Also in order to protect'capacitorCl "from deleterious surges or opencircuiting 0f resistors R1 =or 'Rz, a 'safety' circuit is 'included 'inthe milliamperage preseiector. This safety circuit includes twovoltageregulator tubes- 43'and with said tubes being connected in serieswith 'each other across the "stabilizer terminal "ll/1A1 and ground.Resistors R -arid"=l'tq 'shuntthe respective-tubes "43 and 64" toequally *dividethe voltage thereacross. 'These tubes'accordinglyare soarranged as tobreakfiownshoul'd the voltage across capacitor-C1 exceedabodt300 volts. "These voltage regulator tubes 43'and M' are connectedor interlocked into -the'"X-rayn push-button circuit, asindica'tedby thelegendfbyvirtue of the safety contacts contained within the tube base.In-other-wor'ds'two pins in the voltage regulator tube"base areconnected together and the "two tubes connected 'in series with eachother and connected to the 'X-ray On circuit at terminals 4'5*and"43extendingfrom' the tubes "Aland M. In additionto protecting capacitorflifrom overvoltage. "the tubes 43 and M. protect'the "entire equipment inthe sense *that theymust be "insorted in their respective "sockets torender the fiiter in the *form of a capacitor 49 is Fshunte'd"acrosstlre inductancedfito remove'anypossibility of-thdGOcycle componentgetting-through inductanc'e fil'dueto any i-distrihuted capacitanceexisting in' this inductance "kl. Air-additional *capa'citor.E'Gismonnected *across tlie stahiiizer'supply' conductors G1 andMArtofurther filter 'theen'- cu it and across which the output vo'ltageofthe filter circuit appears. This filter circuit thus removes thecurrent componentj due to cable capacitance of the conductors and"current due to She -'distributed capacitance in the high +tensienwindings:and'accordingly applies "to the stabilizer a --volta-ge=whicheis only dependent-oaths actual tube current.

- stabilizer :is:-.a1so :Prcv-ided irrith what" is terme'dwa fneonidiscriminator :wlfichrccmpris'es a neonitubeifil in: seriesxwi thonetofz'ithezstabilizer conductors together with :a fixed: resistancer53 and variable; resistance? .14 connected across, the stab'ilizersupply:conductorsw G1 and F'MA1. x The pun-pose of 'rthis neonediscriminatoris to provide fiurtheramplification for proper control of z'thestabilizer-system. .Sin cetheneon tube 52 115 a colds'cathode'tube andso constructed thati th'e voltage :rdr'opacrossthe tube is constant,regardless :of :current passing through the tube, :athe addition ofresistance "53and variablecresistance 54' -wi'llregulatei theidivisionof voltage.

' :1'Ilhisemay-fbe' betterappreciated by assuming certain fvaluesof'voltage s-uch,x'fors-example, as L00.woltsabeingfiimpressed acrossthe neon lamp andrithe 'erid :ofrvariablexresistance 54. "Due-t0thevoonstant voltage :drop characteristic 10f the neon lamp :52, thevoltage- -drop thereacrossrrwill approximate volts, leaving'a-voltagedrop .of approximately .30-volts across the resistances-53 amid-i4.Ifnowthevoltage-across the terminals is increased from volts to volts,the voltagea rdrop .across'the neon lamp still remains at '70 volts butincreasing thevoltage dropacross the resistancestfiuand r54 to-40'volts. Thus, only a ten percent "increase in input voltagehasfioccurred, but thevoltage drop across resistances 53 and 54 hasincreased thirty-three percent, therebyygiving an approximate gain inthe amplifications-factor;- of three which facilitates the :need forvoltage gain in: the Xg-ray :tube circuit The magnitude of the currentflowing in the neon discriminatorcircuit is controlled bythe'resistan'ces2-53rand54 with the adjustabi-lityqof the latterenabling closer tuning oflthe filter circuituconsisting of theinductanees'tl-and '48' together-rwithithe capacitors 49 and 50byefiectively. clipping the peak of the voltage wave present atzlngherkilovoltage-wvhile maintainingthe milliamperage constant for allpractical purposes at:a11-kilovo1tage settings.

The output of the neon discriminator 1 circuit "connects to the controlgrid of electronic control'tube 3'! through a battery 55 andaressistancelelement 55. 'Thiszbattery 55. is provided to select theoperating point Withregard to the characteristic curves of the tube .31,and :isnot subjectto normal deterioration effects since no.currenti'istaken' from the battery 55 itself during-operation, with thebattery merely adding to the output voltageoof the neon discriminator.circuit,.thereby varying the plate'current in'the control tube 31.

In order to supply the 350 volts D. C.1tothe D. --'C. V winding 35 :of.the saturablereactor 135, a power transformer 1151 is provided whichhas" its primary winding :58 connected toappropriate voltage-taps ofauto transformerl1. The-high voltagesecondary windingfiii deliveringapproximately350volts' has its center tap connected-to the cathode-eithecontrol tube 13'! (and thusf'to oneside MA1:of the stabilizer inputi'potential) while the ends of 1' this secondary Windingi'59 .-;conmeetto the respectiveanodes of 'a full vvave rectifier itube $60. Theathermionic cathode of "this rectifiertFl-receives heating currentfrorna low voltage secondary winding .'6:2-provided on the transformer-'51.which delivers about '5' volts; and a second low voltage winding 63delivers about 6.3 volts to the thermionic cathode of control tube 131with the 1 center tap "of this latter sec ondary winding 63 beingconnected to ztheicenter tap#dffitha'liigherrvoltagersecorrdarycfifi andthus tonne sidc ofahe stabilizerizinput suppiyrMal;

The thermionic cathode oi. rectifier 60 connects to one side of the D.C. winding 36 of the saturable reactor 35, as previously mentioned,which, upon closure of a pair of contacts 64 as hereinafter described,thus places the supply of D. C. to'the winding 36 completely under thecontrol of the control tube 31. This D. C. supply for reactor winding36is provided with a choke input filter comprising an inductance 65 andcapacitor 66. Accordingly, the capacitor 66 is charged each half wave ofthe alternating current cycle in well known manner since by virtue ofthe center tapped secondary winding 59, its respective halves are 180out of phase, and the resulting D. C. output voltage appears across oneend of the D. C. winding 36 and the cathode of control tube 31 as abovenoted. The rectifier tube 60 being of the thermionic cathode typerequires a normal delay period of approximately 20 seconds before thetube reaches its normal operating temperature. Advantage is taken ofthis delay characteristic, as hereinafter described, in order to providea desired time delay prior to operation of the control by giving thetube 60 an opportunity to arrive at its proper operating temperature.

Forthe purpose of presetting the filament heat in the X-ray tubecircuit, to approximately the normal operating range as will be requiredfor the particular milliamperage selected and to also make themilliamperage stabilizer independent of line voltage variations, apreset circuit includin a voltage regulator tube 61 is provided forcontrolling the voltage supplied to the screen grid of control tube 31.This preset circuit includes the voltage regulator tube 61 having itscathode connected to one side of capacitor 66 and thus to the negativeside of the D. C. output of the previously described filter circuitwhich includes the capacitor 66. The anode of voltage regulator tube 61is connected through a' resistance 68 to the opposite or positive sideof the choke filter circuit and hence to one side of the D. C. winding36, and such anode is also connected through an adjustable resistance 69to the screen grid of control tube 31. The previously mentioned contacts64 are connected to two pins on the base of the voltage regulator tube61 which provides a safety feature by requiring this tube to be insertedin its socket before the D. C. power supply becomes operative. Thissafety feature precludes the possibility of too high a screen gridvoltage being applied to control tube 31, which might otherwise be abovethe safe continuous screen grid voltage recommended by the tubemanufacturer.

As hereinbefore mentioned, the control tube 31 has such characteristicsthat if the screen-grid voltage is held constant the plate currentthrough the control tube 31 is relatively independent of line voltageand. hence variations in the latter have substantially no eiiect on theplate current. However, for the purpose of controlling the screen gridvoltage of control tube 31 to enable the filament of the X-ray tube tobe properly preset at the termination of a given exposure, a relay 10 isprovided. The winding of this relay is controlled by the X-ray Oncontrol circuit. This relay 10 is provided with a pair of contacts12which upon closure short-circuit the adjustable resistance 69 and alsoprovided with a set of contacts13 connected to the motor of timer 26 sothat the latter operates as soon as the X-ray tube is energized.

In the absence of provision to the contrary operation oithemilliamperage stabilizer is de- 8 pendent on;the.magnitude of thevoltage EP'. plied to the control grid of the control tube 31.Accordingly, when the voltage is removed from the control grid of tube31, the grid bias will then drop to the valueof the bias battery iii.This will increase the; plate current in tube 31 and decreases theeffective impedanceof the saturable reactor 35 and the filament heat ofthe X-ray tube will be increased, which produces an undesirablecondition in that current through the X-ray tube reaches relatively highvalues upon resumption of another exposure. It is to prevent suchundesirable result that provisions are made to control the voltage onthe screen grid of control tube 31.

Prior to initiation of an X-ray exposure, the relay 1B is inoperativeand the screen grid is supplied with a voltagethrough resistances 68 and69. Since the voltage drop across these resistances are additive; avoltage somewhat lower than that required for normal operation issupplied to the screen grid of control tube 31, and with this reducedscreen-grid voltage the plate current in the control tube is reduced.Under conditions of reduced plate current, the impedance of thesaturable reactor 35 will be raised thereby lowering the filament heatof the X-ray tube cathode as 1011;! as X-rays are not being produced.

When the X-ray On push button is operated, relay 1B is energized andcloses its contacts 12, which thus short-circuits the resistance 69-, aspreviously mentioned, and raises the voltage supplied to the screen gridof control tube 31 to its normal operating voltage of about volts. Thisirnmediatelyraises the plate current of control tube 31 and reduces thereactance of the saturable reactor 35, which in turn increases thefilament heat of'the X-ray tube cathode during the exposure and untilthe latter is terminated b the timing dev ce 25 or by depression of theX-rav Off" push button. Thus the voltage regulator tube. to ether withits circuit as above described. performs the desired function ofmaintain ng relativelv stable operation of the milliamnerage stabilizerregardless of line voltage fluctuations and provides a desirable presetcharacteristic for t e X-ray filament circuit by autom ticallv reducinthe filament heat of the X-rav tube cathode at the termination of anX-ray exposure andraising the filament heat upon initiation of a givenexposure.

As hereinbefore mentioned, the control tube 31 is preferably one havinga high value of transconductance so as to give optimum performance in amilliarnperage type stabilizer. Such high value of transconductanceenables the milliamoerage stabilizer in general to have sufiicient ga nto overcome eifects of increased filament voltage, increased temperaturein the filament circuit which affects filament resistance and incre s dkiiovolta we. Howe er. because of the act th t a hi h ain is obtained. afeedback condition arises between the milliamperage stabilizer and theX-ray tube. This condition causes a ra id osc llatio in the mi liammetercircuit and undesirable characteristics are obtained. In ns'inc acontrnltub'e ha ing a high value of transc nductance to obtain its highvolta e again. as above-noted, it would be im ossible to successfullvcompensate For the efiects o increased filament voltage. increased temerature n the filament circuit which affects filament resistance andincreased kilovoltage without overcoming this fe dback condition. 7

To accomplish the latter, a feedback circuit is provided comprisingafee'd back transformer 14-, the primary winding: 15' of which isconnected across the resistance 3i included in series with the primarywinding 28" of the heating transformer l6 for the'cathode l2'of X-raytube and the A. C. windings 33' and 34 of reactor 35. The primarywinding Hi-of the feedback transformer is thus subjected to the voltagedrop across the resistance 3| which is thus proportional to the currentflowing in-the primary winding of the low tension cathode heatingtransformer Hi. This voltage across primary winding boosted by acentertapped step-up secondary winding It whichis connected to afullwaverectifier circuit comprisinga rectifier- Ti; capacitor l8 andresistor 19; The output from the-'step-up windinglfi is accordinglyimpressed across resistor 19 with the filter capacitor 18- sl'iunt'edacross the resistor 19 toadd sufficient time duration to thevoltagepulsesappearing acrossthe resistor 19-- which averages out over ahalf cycle since the constants of this capacitor 18 and resistor 155 areso chosen that the voltage durationis /120 of a second.

The voltage which appears across resistor 19 is in turn cap'acitativ'elycoupled to the previously mentioned resistance element 56 through acapacitor 80. Under normal conditions, that is a condition in whichoscillations do not occur in the X-ray tubemilliampera'geand henceinthe' filament heating circuit for the X'-ray tube oath; ode, thevoltage drop across resistor'lfl remains" constant and is blocked'b'y'capacitor 80 from in any way affecting the control grid of control tube31. On theother hand, should oscillations occur in the filament circuitand in turn the milliamperage of the" X-ray tube, a pulsating of' thecontrol tube 3 lwheneveroscillationsde velopin the filament circuit,"since the applica tion of such negative'pulse to the grid'iminedt This"is accomplished by selecting the polarity of the voltage acrossthe'capa-citor 80 so -that a negative" pulse is supplied to the'gridately result in dampening of the oscillations in th'fllanient circuit. I

As" previously referred to, advantage is'take'n of the time delaycharacteristic ofthe (rathode of rectifier 6G in reaching an electronemitting temperature to provide-arr additional safety featurewhichenablesthe-entire system to be properly conditioned before anatte'mptismtde to operate the apparatus-to produce -X-rays3 To this end,the previously mentioned relay 2'! "hasits winding connected across the'capacitor '66: and isthus subjected to the voltage across thelatter. IAccordingly, since the voltage across capacitor 66*does' not appearuntil-the expiration of the time-delay required b'y 'the cathodeofrectifier ously mentioned contacts 25; Closure of these lattercontacts 25 accordingly "conditions the" X-ray On control'circuit 'for'operation upon depression'of the -X-ray O-n push button, since thecontrol is' theri -=completed except at 10 thc poirit-oiiacltisure hycreamer. Upon dpres..-; si'o 1 of the-X-ray ow'push buttomthe winding-r.ofrelay 1'0="lcecom"e'senergized by completion of the -K ra Qh controlcircuit and closes its pair of-= contacts 12* and 13. Whencontacts -12are thus close theieslstance 69 is-short-cir-- cuit'ed to raisethevoltage supply to the screen grid of 5 control 31-, previouslydescribed, and-simultaneously therewith'closure of contacts 13* com1etesa'circuit-to the timer 26 starting the 'latter inoperation:

At the same-"tiine -depression of the X-ray On p=ush-buttoncompletesacircuit to the winding; ofmagnetic'ally operated switch 23energizing-the latter with attefidant closure' of its contacts 82 and-33. r@ontacts':82--'accordingly causeenergization of :the primarywinding 22-of high tensiontrans'former Qahd the'generatiQnof the X-rays'forthe period of time for-iwhich: the timer .26 has been preset,-'-oruntil=' the X-ray Off push abutton-- isdepressed whileclosurewofcontacts 83, completes-aholding: circuit for the Windingsof'.magnetically operated' switch 23, thus enablingthe-X-ray On? push'buttonto be immediately released following initial depression thereof Alsocontacts-83: complete aholding circuit .for the winding-P of relay 10-so that the contacts I2. and 13+: of the latter. maintaintheshort-circuit condition of;resistance'69-'and' the timer 26 inoperation hntil the exposure .is completed by, openingoisthehormallyclosed contacts ofthe. timer-2t, oropening *of the controlcircuit by depressionaof. the ,fX-ray Off, push :button. .It. canthus-'beseen that the X-ray On pushibut-v ton v,is, therefore.inoperativev to complete the, fX1ray.,ncontrol circuit; to energizerelays 23 and H1, which thus precludes the generation of X-rayauntilrelay 21is energized after expiration of -.=the,above noted time-delayand closure of its contacts 25:

By-reference .tothe drawing, it will be noted; thatv thefX-ray Oncontrol, circuit receives energyiromtaps on-theautotransformer IT. For;example; a conductor, extends from a tap of, autotrah'sformer'. l.'l to.one side, of winding ,of magnetically, operated-switch 23, and this sameconductor also extends to'onerside of the Wind ng; of relay;.18,maswell: as to one contact thereof. The other. side of. this-controlcircuit extends; fromranother tap -of the autotransformer l'l and by: aconductor-85: through the pi s of the v lta e regulator tubes-Hand 44;in series; and through the icontacts 250i the relay; 2], and thenthrough normally closed" X ray: Off? push, button, the, normallyclosed'contacts of the timer 26, and to, one-side of the, ,X-iray-Onpush button as Well; aszi'one of thecontacts -83 of ma n al y 0 eratedswitch:23. l ,i

This control circuit is: thus" initi 126. the'vcontactsza: and thenormally open ii-ray ;v

Orr-'1? push-'button Upon initial energization oj On pu h -b o fs orth uo me ta y pression of the latter, the X-ray Onfl contrgl circuit"'is' icompleted in themanner abovedescribed-"1? .-It: w il1: :also 'rbe actedthat the magnetieaiii,

operated switch l8, which to all intents and-pure poses may beconsidered as the main line switch, is energized by depression of pushbutton 86, which thus completes a circuit to the winding thereofdirectly from the commercial source of supply'of 200-250 volts or suchwinding may be energized from taps of autotransformer H as desired. Uponenergization of the winding of magnetically operated switch l8, threesets of contacts are closed, two of such sets completing the commercialsource of supply to the autotransformer ,l'l, while the other set ofcontacts completes a holding circuit to the winding of magneticallyoperated switch l8. When this main line switch is closed, the thermioniccathodes of the various tubes are heated, including the thermioniccathode OI rectifying valves '1 and Hi, although the specific heatingsource therefore is not'shown for simplicity. Once the main line switchl8 has been closed, the entire apparatus is then in condition foroperation in the manner above described and after completion of anexposure, the main line switch is de-energized when desired bydepression of a normally closed push button Bl.

The foregoing milliamperage stabilizer has beenshown and described withwhat is termed half-wave equipment, which means that the rectifiedalternating current is supplied to the X-ray tube during half-waves ofthe alternating current cycle. However,- suchstabilizer can be just asreadily employed with a full wave bridge circuit by inserting themilliamperage stabilizer in the D. C. circuit of the milliammeter andutilizing a rectifier bridge arrangement. wave arrangement has not beenshown for the sake of simplicity.

It thus becomes obvious to those skilled in the art that a milliamperagestabilizer is herein provided which maintains the current through theX-ray tube substantially constant during an exposure. Moreover, suchstabilizernot only compensates for fluctuations of voltage in the sourceof supply but also compensates for increased filament voltage, increasedtemperature in the filament' circuit which affects filament resistance,and increased 'kilovoltage.

' By the utilization of a filter choke net-work, oscillations within thehigh voltage circuit are precluded from affecting the stabilization anda discriminator arrangement advantageously amplifies the current so asto better control the latter through a control tube having a high valueof conductance. Also to prevent a feedback, a

circuit is provided, thereby eliminating any tendency of the tubecurrent to fluctuate. Moreover, safety features are provided so that theapparatus is precluded from operating to produce X-rays until certainvoltage regulator tubes have been inserted in their appropriate sockets,and in addition the stabilizer prevents exposures from being initiateduntil the thermionic cathodes of the various tubes have reached theirelectron emitting temperature, thus eliminating any possibility of anover voltage being impressed across any part of the system.

Although one specific embodiment of the present invention has been shownand described, it is to be understood that other modifications thereofmay be made without departing from the spirit and scope of the appendedclaims.

I claim:

'1. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh Such full I 12 tension transformer'provided with a primary windingconnected toa commercial source of supply subject to line voltagefluctuations and having its secondary winding connected through. arectifying arrangement to said electron discharge device for supplyingunidirectional. cur

rent thereto, a source of alternating current electrical energy subjectto changes in resistance with an attendant variable current componentfor heating the cathode of said electron discharge device, a saturablereactoroperatively connected with said source of heating energy andoperable upon the imposition .of a unidirectional component thereupondiffering from the current component of the heating source to controlthe flow of heating current to the cathode of said electron dischargedewce, adjustable preselector means connected to said excitation circuitand subject to variations in potential thereacross in response tovariations in voltage across and current through said electron dischargedevice, as well as line voltage fluctuations and change in resistance inthe source of heating energy for saturable reactor and said adjustablepreselector means and operable upon a variation in the potential acrosssaid adjustable preselector means to cause the imposition of aunidirectional current component upon said saturable reactor differingfrom the current component of said heating source accompanied by avariation in the heating current supplied to the cathode of saidelectron discharge device inversely to variations in the voltageacrossand current through said electron discharge device as well as linevoltage fluctuations and change in resistance in the source of heatingenergy for the cathode of said electron discharge device.

2. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a source of alternating currentelectrical energy for heating the cathode of said electron dischargedevice normally subject to a variable current component, a saturablereactor operably connected with said source of heating energy andoperable upon the imposition of a, unidirectional component thereupondiffering from the current component of the heating source to controlthe flow of heating current to the cathode of said electron dischargedevice, adjustable preselector means connected to said excitationcircuit and subject to varia-.

tions in potential thereacross in response to variations in dischargecurrent through said electron discharge device, and said adjustablepreselector means including a filter arrangement to remove the currentcomponent due to distributed capacitance in said excitation circuit andto make the voltage across said adjustable preselector means dependentonly on the actual current supplied by said excitation circuit to saidelectron discharge device, and electrical circuit means operativelyconnected with said saturable reactor and said adjustable preselectorwith both said aforementioned means and operable upon a variation in thepotential across said adjustable preselector means to cause theimposition of a current component upon said saturable reactor differingfrom that of said heating source accompanied by a variation in theheating current supplied 'to-the' cathode of: said'electrofi dischargedevice inversely to variations in-the discharge current flowing throughthe latter.

3; The combination of an electron discharge device provided with'acathodeto be heated, an excitation circuit for said device including ahigh tension transformer having its secondary wind ing'connected througharectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, source of alternating current electricalenergy 'for' heating the cathode of said-electron discharge devicenormally subject to'a variable current component, a saturablereactoroperatively connected with said source of heating energyandoperable'upon the imposition of a unidirectional component thereupondiffering from the current component of the heating source to controlthe'flow ofheating current to the cathode of said electron dischargedevice, adjustable preselector' meansconnected to said excitationcircuit and subject to variations in potential thereacross in responseto variations in discharge current through said electron dischargedevice, and said adjustable preselector means including a filterarrangement comprising two inductanceelements in series relationtogether with a capacitor shunting one of said inductance elements and acapacitor in shunt with said adjustable preselector means and operableto remove the current component due to distributed capacitance in saidexcitation circuit and to make the voltage across said adjustablepreselector means dependent only on the actual current supplied by saidexcitation circuit to said electron discharge device, and electricalcircuit means operatively connected with said saturable reactor and saidadjustable preselector means and operable upon a variation in thepotential across said adjustable preselector means to cause theimposition of a current component upon said saturable reactor difieringfrom the current component of said heating source accompanied by avariation in the heating current supplied to the cathode of saidelectron discharge device inversely to variations in the dischargecurrent flowing through the latter.

4. The combination of an'electron discharge device provided withacathode to be heated, an excitation circuit for said device including ahigh tension'transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a, source of alternating currentelectrical energy for heating the cathode of saidelectron dischargedevice normally subject to a variable current component, a saturablereactor operatively connected with said source of heating energy andoperable upon the imposition of a unidirectional component thereupondiffering from the current component of the heating source to controlthe flow of heating current to the cathode of said electron dischargedevice, adjustable preselector means connected to said excitationcircuit and subject to variations in potential thereacross in responseto variations in discharge current through said electron dischargedevice and including a voltage divider discriminator arrangement foramplifying the voltage across said adjustable preselector means, andelectrical cir-i "cuit means operatively connected with said-saturablereactor and said adjustable preselector means and operable upon avariation in the potential across said adjustable preselector meansstd-cause the imposition of a current component upon said saturableireactor differing? from'ithe current component of said heating sourceac;- companied by a variation in the heatingcurrent supplied to thecathode of said electron dis.- charge device inversely to variations inthe discharge current flowing through the latter.

5. The combination of an electron discharge device provided with acathode to be heated,,an excitation circuit for said device includinge;high tension transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a source of alternating currentelectrical energy for heating the cathode of said electron dischargedevice nor.- mally subject to a variable current component, a

saturable reactor operatively connected with said source of heatingenergy and operable upon the imposition of a unidirectional componentthereupon differing from the current component of the heating source tocontrol the flow of heating current to the cathode of said electrondischarge device, adjustable preselector means connected to saidexcitation circuit and subject to variations in potential thereacross inresponse to variations in dis-charge current through-saidelectronvdischarge device and including a voltage divider discriminatorarrangementcomprising a discharge device having a constant: voltage dropthereacross in series with said adjustable preselector means and aresistor in parallel with thelater foramplifying the voltage across saidadjustable: preselector means, and electrical circuit means operativelyconnected with said saturable reactor and said adjustable preselectormeans and operable upon a variation in the potential across saidadjustable preselector means to cause the im position of a currentcomponent upon said saturable reactor differing from the current component of said heating source accompanied by a variation in the heatingcurrent supplied to-the cathode of said electron discharge deviceinversely, to variations in the discharge current flowing through thelatter.

6. The. combination of an electron discharge deviceprovided with acathode to be heated, an excitation circuit for said deviceincludingahigh tension transformer having its secondary winding connected'througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a sourceof alternating currentelectrical energy for heating the cathode of said electron dischargedevice normal ly subject to a variable current component, a saturablereactor operatively connected withsaid source of heating energy andoperable .upon the imposition of a unidirectional component thereupondiffering from the current component ofthe heating source to control theflow of heating cur-- rent to the cathode ofsaid electron dischargedevice,. a source of unidirectional electrical energy for supplying theunidirectional component to said saturable' reactor, adjustablepreselector means connected to said excitation circuit and subject tovariations. in potential thereacross in response to variations indischarge current through said electron discharge device, electricalcircuit means operatively connected with said saturable reactor and saidadjustable preselector means and operable upon a variation in the.potential across said adjustable pres elector means to cause said sourceof unidirectional electrical energy ,to' impose" the aforementioned;current component upon said saturable reactordiffering 75.:from1 the:current component.- of said heating tource acor'ripa'iiid by a variationin the heating current supplied to the cathode of said electrondischarge device inversely torvariations in the discharge currentflowing through the latter, and means connected to said source ofunidirectional electrical energy and operable by the latter to cause atime-delay before said excitation circuit becomes effective toenergizesaidelectron discharge device.

7. The combination 01 an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its'secondary wind:- ing connectedthrough a rectifying arrangement to said electron discharge device forsupplying unidirectional current thereto, a source of alternatingcurrent electrical energy for heating the cathode of said electrondischarge device normally subject to a variable current component, asaturable reactor operatively connected associated with said source ofheating energy and operable upon the imposition of a unidirectionalcomponent thereupon difiering from the current component of the heatingsource to control the flowof heating current to the cathode of saidelectron discharge device, a source of unidirectional electrical energyfor supplying the unidirectional component to said saturabie reactor,adjustable preselector means connected to said excitation circuit andsubject to variations in potential thereacross in response to variationsin discharge current through said electron discharge device, electricalcircuit means operatively connected with said saturable reactor and saidadjustable preselector means and operable upon a variation in thepotential across said adjustable preselector means to cause said sourceof unidirectional electrical energy to impose the aforementioned currentcomponent upon said saturable reactor differing from the currentcomponent of said heating source accompanied by a variation in theheating current supplied to the cathode of said electron dischargedevice inversely to variations in the discharge currentflowing throughthe lat- ;ter, and a relay operatively connected to said source ofunidirectional electrical energy and en- .ergizable by the latter tocause a time-delay be- ;fore said excitation circuit becomes eifectiveto energize said electron discharge device.

8. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingcurrent to the cathode of said electron discharge; device, a source ofunidirectional electrical energy for supplying the unidirectionalcomponent to said saturable reactor, adjustable preselector meansconnected to said excitation circuit and subject to variations inpotential thereacross in.-

response to variations in discharge current through said electrondischarge device including an electronic control tube having a highvalue of transconductance to cause a variation of relative- ,ly greatmagnitude in plate current through said.

control tube in response to relativelysli ghtvarizji tions in gridvoltage supplied thereto and oper-' able to control the supply ofunidirectional en ergy from said source to said saturable reactor;electrical circuit means operatively connected with said saturablereactor and said adjustable preselector means and operable upon avariation in the potential across said adjustable preselector means tocause operation of said control tube with the imposition of a currentcomponent from said unidirectional source upon said saturable re actordiffering from the current component of said heating source accompaniedby a variation in the heating current supplied to the cathode of saidelectron discharge device inversely to varia tions in the dischargecurrent flowing through the latter, and a relay operatively connected tosaid source of unidirectional electrical energy and energizable by thelatter to cause a time-delay before said excitation circuit becomeseffective to energize said electron discharge device.

9. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a source of alternating currentelectrical energy for heating the cathode of said electron dischargedevice normally subject to a variable current component, a saturablereactor operatively connected with said source of heating energy andoperable upon the imposition of a unidirectional component thereupondiifering from the current component of the heating source to controlthe flow of heating current to the cathode of said electron dischargedevice, adjustable preselector means connected to said excitationcircuit and subject to variations in potential thereacross in responseto variations in discharge current through said electron dischargedevice, and including an electronic control tube provided with a controlgrid and a screen grid and having a high value of transconductance tocause a variation of relatively great magnitude in plate current throughsaid control tube in response to relatively slight variations in gridvoltage supplied thereto, and electrical circuit means operativelyconnected with said saturable reactor and said adjustable preselectormeans and including a source of direct current power for said controltube to cause operation of the latter with attendant imposition of acurrent component upon said saturable reactor differing from the currentcomponent of said heating source accompanied by a variation in theheating current supplied to the cathode of said electron dischargedevice inversely to line voltage fluctuations effecting the dischargecurrent flowing through said electron discharge device, and meansconnected to said electronic control tube and operable to cause thelatter to-become operable and cause an attendant automatic increase inthe current supplied to the cathode of said electron discharge devicesimultaneously with energization of said electron discharge device bysaid excitation circuit and to automatically cause a decrease in thecurrent supplied to the cathode of said electron discharge devicesimultaneously with deenergization of said electron discharge devicefrom said excitation circuit.

10. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh thereupon difierin from the current component of the heating sourceto control the flow of heating current to the cathode of said electrondischarge device, adjustable preselector means con.- nected to saidexcitation circuit and subject to variations in potential thereacross inresponse to variations in discharge current through said electrondischarge device, and including an'electronic control tube provided witha control grid and a screen grid and having a high value oftransconductance to cause a variation of rela tively great magnitude inplate current through said control tube'in response to relativel slightvariations in grid voltage supplied thereto,electrical circuit meansoperatively connected with said saturable reactor and said adjustablepreselector means and including a source of direct current power forsaid control tube and a voltage regulator tube for controlling the powersupplied 'by said direct current source to the screen grid of saidcontrol tube to causeoperation of the later with attendant imposition ofa current component upon said saturable reactor differing from thecurrent component of said heating source accompanied by a variation inthe heating current supplied to the cathode of said electron dischargedevice inversely to line voltage fluctuations affecting the dischargecurrent flowin through said electron discharge device, and meansconnected to said electroniccontrol tube and operable simultaneouslywithenergization of said electron discharge device by said excitationcircuit to cause an increase in the potential supplied to the grid ofsaid electronic control tube with an attendant automatic in crease inthe current supplied to the cathode of said electron discharge deviceand operable to automatically cause a decrease in the potential suppliedto the grid of said electronic control tube with an attendant decreasein the current supplied to the cathode of said electron discharge deviceupon deenergization of said electron discharge device.

11. The combination of an electron discharge device provided with acathode to be heated, an

excitation circuit for said device including a high tension transformerhaving its secondary winding connected through a rectifying arrangementto said electron discharge device for supplying unidirectional currentthereto, a source of alternating current electrical energy for heatingthe cathode of said electron discharge device normally subject to avariable current component, a saturable reactor operatively connectedwith said source of heating energy and operable upon the imposition of aunidirectional component thereupon differing from the currentcompon'entof the heating source to control the flow of heating current to, thecathode of said electron discharge device, adjustable preselector meansconnected to said excitation circuit and subject to variations inpotential thereacross in response to variations in discharge currentthrough said electron discharge device, and including an electroniccontrol tube provided with acontrol grid and a screen grid and having ahigh value of transconductance to cause a variation of relatively Jgreatmagnitude in plate current through said control tube in response torelatively slight variations in grid voltage supplied thereto,electrical circuit means operatively connected with said saturablereactor and said adjustable preselector means and including a source ofdirect current power for said control tube and. a voltage regulator tubefor controlling the power sup-- plied by said direct current source tothe screen grid of said control tube to cause operation of the latterwith attendant imposition of a current component upon said saturablereactor differin from the current component of said heating sourceaccompanied by a variation in the heating current supplied to thecathode of said electron discharge device inversely to line voltagefluctuations eifecting the discharge current flowing through saidelectron discharge device, a resistance in series with said voltageregulator tube, and a relay operable simultaneously with energiz'ationof said electron discharge device by said excitation circuit toshort-circuit said resistance and cause an increase in the potentialsupplied to the grid of said electronic control 1 tube with an attendantautomatic increase in the current supplied to the cathode of saidelectron discharge device, and operable to connect said resistance inseries with said voltage re ulator tube and cause an automatic decreasein the cur rent supplied to the cathode of said electron dischargedevice upon deenergization of said electron discharge device.

12. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a source of alternating currentelectrical energy for heating the cathode of said electron dischargedevice norma1- 1y subject to a variable current component, a

saturable reactor operatively connected with said source of heatingenergy and operable upon the imposition of a unidirectional componentthereupon differing from the current component of the heating source tocontrol the flow of heating current to the cathode of said electrondischarge device, adjustable preselector means connected to saidexcitation circuit and subject to variations in potential thereacross inresponse to variations in discharge current through said electrondischarge device, and including anelectronic'control tube provided witha control grid anda screen grid and having a high value oftransconductan'ce to cause a variation of relatively great magnitude inplate current through said'cont'rol tube in re sponse to relativelyslight variations in grid Voltage supplied thereto, and electricalcircuit means operatively connected with saidsaturable reactor and saidadjustable preselector means and including a "source of direct currentpower for said control tube and a voltage regulator tube for controllingthe power supplied by said direct current source to the screen grid ofsaidcont'rol tube to cause operation of the latter with attendantimposition of a current component upon said saturable reactor differingfromthe current component of said heating source accompanied by avariation in the heating current supplied to the cathode of saidelectron discharge deviceinversely to line voltage fluctuationseffecting the dis- 19 charge current flowing through said electrondischarge device.

13. The combination of an electron discharge device provided with acathode to be heated, an

excitation circuit for said device including a high tension transformerhaving its secondary winding connected through a rectifying arrangementto said electron discharge device for supplying unidirectional currentthereto, a source of alternating current electrical energy for heatingthe cathode of said electron discharge device normally subject to avariable current component, a saturable reactor operatively connectedwith said source of heating energy and operable upon the imposition of aunidirectional component thereupon differing from the current componentof the heating source to control the flow of heating current to thecathode of said electron discharge device, adjustable preselector meansconnected to said excitation circuit and subject to variations inpotential thereacross in response to variations in discharge currentthrough said electron discharge device, and including an electroniccontrol tube provided with a control grid and a screen grid and having ahigh value of transconductance to cause a variation of relatively greatmagnitude in plate current through said control tube in response torelatively slight variations in grid voltage supplied thereto, andelectrical circuit means operatively connected with said saturablereactor and said adjustable preselector means and including a source ofdirect current power for said control tube and a voltage regulator tubefor controlling the power supplied by said direct current source to thescreen grid of said control tube to cause operation of the latter withattendant imposition of a current component upon said saturable reactordifierlng from the current component of said heating source accompaniedby a variation in the heating current supplied to the cathode of saidelectron discharge device inversely to line voltage fluctuationseffecting the discharge current flowing through said electron dischargedevice, and voltage limiting means connected to said voltage regulatortube and short-circuited simultaneously with energization of saidelectron discharge device by said excitation circuit to cause saidvoltage regulator tube to vary the potential supplied to the screen gridof said electronic control tube with an attendant'automatic increase inthe current supplied to the cathode of said lectron discharge device andthe short-circuit condition of said Voltage limiting means being removedto automatically decrease the current supplied to said cathode upondeenergization of said electron discharge device.

14. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its secondary winding connected througha rectiiying'arrangement to said electron discharge device for supplyingunidirectional current thereto, a source of alternating currentelectrical energy for heating the cathode of said electron dischargedevice normally subject to a variable current component, a saturablereactor operatively connected with said source of heating energy andoperable upon the imposition of a unidirectional component thereupondiffering from the current component of the heating source to controlthe flow of heating current to the cathode of said electron dischargedevice, adjustable preselector means connected to said excitationcircuit and subject to variations in potential thereacross in responseto variations in discharge current through said electron dischargedevice, and including an electronic control tube having a high value oftransconductance to cause a variation of relatively great magnitude inplate current through said control tube in response to relatively slightvariations in grid voltage supplied thereto, and electrical circuitmeans operatively connected with said saturable reactor and saidadjustable preselector means and operable upon a variation in thepotential across said adjustable preselector means to cause theimposition of a current component upon said saturable reactor differingfrom the current component of said heating source accompanied by avariation in the heating current supplied to the cathode of saidelectron discharge device inversely to variations in the dischargecurrent flowing through the latter.

15. The combination or an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a source of alternating currentelectrical energy for heating the cathode of said electron dischargedevice normally subject to a variable current component, a saturablereactor operatively connected with said source of heating energy andoperable upon the imposition of a unidirectional component thereupondiffering from the current component of the heating source to controlthe flow of heating current to the cathode of said electron dischargedevice, adjustable preselector means connected to said excitationcircuit and subject to variations in potential thereacross in responseto variations in discharge current through said electron dischargedevice, electrical circuit means operatively connected with saidsaturable reactor and said adjustable preselector means and operableupon a variation in the potential across said adjustable preselectormeans to cause the imposition of a current component upon said saturablereaction differing from the current component of said heating sourceaccompanied by a variation in the heating current supplied to thecathode of said electron discharge device inversely to variations in thedischarge current flowing through the latter, and feedback meansconnected to the source of heating energy for the cathode of saidelectron discharge device and to said adjustable preselector means andoperable to apply a pulsating voltage to said adjustable preselectormeans upon the occurrence of oscillations in said excitation circuit.

16. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a source of alternating currentelectrical energy for heating the cathode of said electron dischargedevice normally subject to a variable currentcomponent, a saturablereactor operatively connected with said source of heating energy andoperable upon the imposition of a unidirectional component thereupondiffering from the current component of the heating source to controlthe flow of heating current to the cathode of said electron dischargedevice, adjustable preselector means connected to said excitationcircuit and subject to variations in potential thereacross in responseto variations in discharge current through said electron dischargedevice, electrical circuit means operatively connected with saidsaturable reactor and said adjustable preselector means and operableupon a variation in the potential across said adjustable preselectormeans to cause the imposition of a current component upon said saturablereactor differing from the current component of said heating sourceaccompanied by a variation in the heating current supplied to thecathode of said electron discharge device inversely to variations in thedischarge current flowing through the latter, and feedback meanscomprising a transformer having its primary winding associated with thesource of heating energy for the cathode of said electron dischargedevice and the secondary winding thereof connected to said adjustablepreselector means and operable to apply a pulsating voltage to saidadjustable preselector means upon the occurrence of oscillations in saidexcitation circuit.

17. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a source of alternating currentelectrical energy for heating the cathode of said electron dischargedevice normally subject to a variable current component, a saturablereactor operatively connected with said source of heating energy andoperable upon the imposition of a unidirectional component thereupondiffering from the current component of the heating source to controlthe flow of heating current to the cathode of said electron dischargedevice, adjustable preselector means connected to said excitationcircuit and subject to variations in potential thereacross in responseto variations in discharge current through said electron dischargedevice and including a resistor in series with said adjustablepreselector means, electrical circuit means operatively connected withsaid saturable reactor and said adjustable preselector means andoperable upon a variation in the potential across said adjustablepreselector means to cause the imposition of a current component uponsaid saturable reactor difiering from the current component of saidheating source accompanied by a variation in the heating currentsupplied to the cathode of said electron discharge device inversely tovariations in the discharge current flowing through the latter, andfeedback means comprising a transformer having its primary windingassociated with the source of heatin energy for the cathode of saidelectron discharge device and the secondary winding thereof connectedthrough a rectifier arrangement across said resistor and operable toapply a pulsating voltage to said adjustable preselector means upon theoccurrence of oscillations in said excitation cincuit.

18. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a source of alternating currentelectrical energy for heating the cathode of said electron dischargedevice, normally subject to a variable voltage component, a saturablereactor operatively connected with said 22 sourceof heating energy andoperable upon the imposition of a unidirectional component thereupondifiering from the current component of the heating source to controlthe flow of heating current to the cathode of said electron dischargedevice, adjustable preselector means connected to said excitationcircuit and subject to variations in potential thereacross in responseto variations in discharge current through said electron dischargedevice and including a voltage divider discriminator arrangement foramplifying the voltage thereacross and a filter arrangement to removethe current component due to distributed capacitance in said excitationcircuit and to make the voltage across said adjustable preselector meansdependent only on the actual current supplied by said excitation circuitto said electron discharge device, and electrical circuit meansoperatively connected with said saturable reactorand said adjustablepreselector means and operable upon a variation in the potential acrosssaid adjustable preselector means to cause the imposition of a currentcomponent upon said saturable reactor differing from the currentcomponent of said heating source accompanied by a variation in theheating current supplied to. the cathode of said electron dischargedevice inversely to variations in the discharge current flowing throughthe latter.

19. The combination of an electron discharge device provided with acathode to be heated, an excitation circuit for said device including ahigh tension transformer having its secondary winding connected througha rectifying arrangement to said electron discharge device for supplyingunidirectional current thereto, a source of alternating currentelectrical energy for heating the cathode of said electron dischargedevice normally subject to a variable current component, a saturablereactor operatively connected with said source of heating energy andoperable upon imposition of a unidirectional component thereupondiffering from the current component of the heating source to controlthe flow of heating current to the cathode of said electron dischargedevice, adjustable preselector means connected to said excitationcircuit and subject to variations in potential thereacross in responseto variations in discharge current through said electron dischargedevice, and said adjustable preselector means including an electroniccontrol tube having a high value of transconductance to cause avariation of relatively great magnitude in plate current through saidcontrol tube in response to relatively slight variations in grid voltagesupplied thereto, a filter arrangement to remove the current componentdue to distributed capacitance in said excitation circuit and to makethe voltage across said adjustable preselector means dependent only onthe actual current supplied by said excitation circuit to said electrondischarge device, and a voltage divider discriminator arrangement foramplifying the voltage across said adjustable preselector means; andelectrical circuit means operatively connected with said saturablereactor and said adjustable preselector means and operable upon avariation in the potential across said adjustable preselector means tocause the imposition of a current component upon said saturable reactordiifering from the current component of said heating source accompaniedby a variation in the heating current supplied to the cathode of saidelectron discharge device inversely to variations in thedischargecurrent flowing through the latter.

gal-gm '23 20. The combination of an electron discharge device providedwith a cathode to be heated, an excitation circuit for said deviceincluding a high tension transformer having its secondary windingconnected through a rectifying arrangement to said electron dischargedevice for supplying unidirectional current thereto, a source ofalternating current electrical energy subject to changes in resistanceand a variable current component for heating the cathode of saidelectron discharge device, a saturable reactor operatively connectedwith said source of heating energy and operable upon the imposition of aunidirectional component thereupon differing from the current componentof the heating source to control the flow of heating current to theoathode cf said electron discharge device, adjustable preselector meansconnected to said excitation circuit and including a filter arrangementto remove the current component due to distributed capacitance in saidexcitation circuit and to make the voltage across said adjustablepreselector means dependent only on the actual current supplied by saidexcitation circuit to said electron discharge device, a voltage dividerdiscriminator arrangement for amplifying the voltage across said means,an electronic control tube havin a screen grid and operable to controlthe imposition of the unidirectional component on said inductive means,feedback means connected to the source of heating energy for the cathodeof said electron discharge device and to said adjustable preselectormeans and operable to apply a pulsating voltage to said adjustablepreselector means upon the occurrence of oscillations in said excitationcircuit; and electrical circuit means 24 operatively connected with saidsaturable reactor and said adjustable preselector means including avoltage regulator tube for controlling the voltage applied to the screengrid of said electronic control tube to cause operation of the latterand operable upon a variation in the potential across said adjustablepreselector means to cause the imposition of a current component uponsaid saturable reactor difiering from the current component of saidheating source accompanied by a variation in the heating currentsupplied to the cathode of said electron discharge device inversely tovariations in the voltage across and current through said electrondischarge device as well as line voltage fluctuations and change inresistance in the source of heating energy for the cathode of saidelectron discharge device.

CHARLES T. ZAVALES.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,893,299 Morrison Jan. 3, 19331,953,889 Mutscheller Apr. 3, 1934 1,961,703 Morrison June 5, 19341,999,736 Morrison Apr. 30, 1935 2,001,567 Case May 14, 1935 2,048,203Spencer July 21, 1936 2,236,195 McKesson Mar. 25, 1941 2,262,044Philpott Nov. 11, 1941 2,319,378 Weisglass May 18, 1943

